It is well known that a single-phase AC motor produces an alternating magnetic field, one pulling first in one direction, then in the opposite direction as the polarity of the magnetic field changes. This is because the single-phase AC motor is energized by a single alternating current source. The major distinction between the different types of single-phase AC motors is how they go about starting the motor in a particular direction. Motor start is usually accomplished by some device or circuit that introduces a phase-shifted magnetic field on one side of the motor shaft, or rotor.
Split-phase motors achieve their starting capability by having two separate windings wound in the motor stator. The two windings are separated such that one winding is used only for starting. The starting, or auxiliary, winding is wound with a smaller wire size having higher electrical resistance than the main windings. Both windings are energized when the motor is started. The starting winding produces a field that appears to rotate. This rotation causes the motor to start. A centrifugal switch then disconnects the starting winding when the motor reaches a predetermined speed.
The winding and centrifugal switch arrangement of a capacitor-start motor is similar to that used in a split-phase motor. In the capacitor-start motor, a capacitor is used in series with the starting winding to produce a phase shift and the appearance of a rotating field. Here again, when the motor approaches a predetermined running speed, the starting switch opens thereby disconnecting the starting winding and the motor continues to run. One starting circuit for use with a motor of this type is disclosed in U.S. Pat. No. 4,622,506. The method and apparatus of the present invention is an improvement of that starting circuit.
Various types of switches, and controls therefor, are also well known in the electrical arts. This includes the mechanical switch and the centrifugal actuator mounted on the motor rotor, as alluded to previously. Mechanical switches of the centrifugal type are subject to problems such as limited life, fatigue, friction, vibration, mounting position, contact wear, among others. Also, the centrifugal switch includes a radial member that blocks axial airflow through the motor, which may impair cooling. This radial member also requires additional room in the motor housing, which may be objectionable in various applications.
In another known start winding disconnect system, Hall effect sensors or pick-up coils are used to detect motor speed to actuate a disconnect switch. This approach may be objectionable because of the requirement of adding an extra element such as a magnet on the motor shaft, and the pick-up coil to sense speed. These extra parts and the assembly required may be cost objectionable.
In another known disconnect system, a timer is started upon initial energization of the motor. When the timer times out, the disconnect switch is actuated to disconnect the auxiliary winding. This approach is not load or speed sensitive, but rather disconnects the auxiliary winding only after a preselected time regardless of motor speed and regardless of load. This approach is limited to dedicated applications where the load on the motor is known beforehand, and the delay time set accordingly. If the load on the motor is increased, the motor speed will not be up to the desired threshold at the noted cutout time. On the other hand, if the load on the motor is decreased, the motor will accelerate faster, and full voltage will be applied across the capacitor for a longer time than is desired, which in turn may damage the motor and/or the capacitor. Capacitor burn-out is a significant problem when reducing the loading of the motor in timed disconnect systems.
Another known approach is to sense current through the main winding and then actuate the disconnect switch at a designated condition. This requires a current sensor in series with the main winding and the start or auxiliary winding, which is objectionable to many manufacturers because of the cost of the extra components and the assembly cost of modifying the circuit and inserting such components in series in the circuit. This approach may also be objectionable due to the extra wattage and heat because current is still flowing through the sensor in the run mode after starting.